In the manufacture of silicon integrated circuits, there is a need to perform a process on semiconductor wafers that will plug vias, form interconnections and make contacts on patterned wafers. The dimensions of the holes or trenches that must be plugged or filled with conductive, usually elemental metallic, material is typically in the submicron regime, or &lt;10.sup.-6 meter. Because of the narrow dimensions and steep stepped sides of these vias and contact holes, physical deposition processes such as sputtering are often unsatisfactory and ineffective.
One convenient and effective process for filling these vias and contact holes with conductive material and for forming contacts has been found to be the chemical vapor deposition (CVD) of elemental tungsten (W). This process, which usually involves a reduction reaction of tungsten hexafluoride vapor (WF.sub.6), results in adequate film conformality in vias and contact holes.
WF.sub.6 can be directly reduced by a substrate itself to cause the tungsten to coat the substrate with a film. This can occur when the substrate reacts with the WF.sub.6, reducing it directly, or where the substrate yields monatomic hydrogen. It has been found, however, that with a substrate such as one of silicon, a limited deposition thickness of tungsten is achieved because the reaction slows substantially as the initial film coats the wafer. When this occurs, further deposition can be achieved only with the availability of another reducing agent, such as H.sub.2 or SiH.sub.4.
The typical CVD tungsten reaction involves use of hydrogen gas (H.sub.2) as a reducing agent for the WF.sub.6 gas. The WF.sub.6 and H.sub.2 gases are usually premixed in an inlet region of a cold wall reactor and then directed onto the surface of a wafer to be coated, which is maintained at an elevated reaction temperature of, for example, 450.degree. C. When the mixed gases contact the wafer at this temperature, the WF.sub.6 and H.sub.2 react producing elemental tungsten (W) which is deposited onto the wafer as a film, and a hydrogen fluoride (HF) byproduct gas that is carried away from the wafer surface by the gas flow within the processing chamber.
However, it has been found that the compatibility of tungsten to materials such as silicon and dielectrics is very poor. Tungsten has been found to initially react with the silicon (Si) of the wafer, producing first a thin film of tungsten silicide (WSi) and creating pits in the underlying silicon layer, resulting in possible defects. The elemental tungsten, however, thereafter adheres to the WSi film.
Adhesion of tungsten to dielectric materials has also been found to be poor. To circumvent the problems of poor adhesion or initial nucleation of the tungsten to the dielectrics and the consumption of silicon from the wafer surface by a W-Si reaction, silane (SiH.sub.4) has been used, at least in the initial phase of the process, for reduction of the WF.sub.6. As a result, any WSi formed along with the tungsten film is supplied from a reaction of WF.sub.6 with SiH.sub.4, and therefore utilizes silicon from the silane rather than from the wafer. Onto this WSi film the W layer is then deposited.
Silane, however, is a hazardous substance and its presence in the manufacturing facility, where permitted by laws and regulations, is undesirable. The substance is toxic, flammable, explosive, and expensive to handle and maintain safely. The use of SiH.sub.4 in W CVD causes silicon to be incorporated into the tungsten films resulting in an increased resistivity of the deposited tungsten film. SiH.sub.4 also reacts with WF.sub.6 at low temperatures, often as low as 15.degree. C., and thus causing deposition of tungsten in undesired locations within the reactor, requiring frequent cleaning of the reactor and thus increasing the reactor down-time and reduced throughput.
Alternatively, adhesion promoting layers have been applied to silicon and dielectrics prior to the tungsten deposition. One such layer is titanium nitride (TIN). Reactively sputtered TiN, CVD TiN and RTP formed TiN have been found to be effective adhesion layers for tungsten deposition on dielectric materials. Nonetheless, unacceptably long incubation times, sometimes in the range of several minutes, have been reported to result before tungsten starts to deposit onto TiN layers when H.sub.2 is used as a reducing agent. With the use of SiH.sub.4 for reduction of the WF.sub.6, the nucleation of W onto TiN films is enhanced and is often almost instantaneous.
TiN itself reduces a WF.sub.6 gas to form a fluoride compound which is not volatile at temperatures below approximately 580.degree. C. Since WF.sub.6 reduction reactions are typically performed at approximately 450.degree. C., the fluoride compound, where it results, can poison the wafer surface inhibiting the H.sub.2 dissociation on the TiN surface, thereby prolonging the W forming reaction.
Accordingly, there is a need in the manufacture of semiconductor devices, for a more effective method of nucleating tungsten, particularly on TiN films on substrates.